This invention relates to methods and products in the utilization and conversion of ash resulting from the incineration of municipal solid wastes, in the form of aggregates useful in asphaltic and portland cement concrete mixes which meet 1990 Federal drinking water standards as defined by the U.S. Environmental Protection Agency.
Municipal solid waste handling and disposal has received substantial attention by agencies of the United States Government as well as by interested environmental groups. For the purpose of this application, municipal solid waste (MSW) is defined as the gross product which is collected and processed by municipalities and governments.
MSW includes durable and non-durable goods, containers and packaging, food and yard wastes, and miscellaneous inorganic wastes from residential, commercial and industrial sources. Examples include newsprint, appliances, clothing, scrap food, containers and packaging, disposable diapers, plastics of all sort including disposable tableware and foamed packaging materials, rubber and wood products, potting soil, yard trimmings and consumer electronics, as part of an open-ended list of disposable or throw-away products. The broad spectrum of MSW content is described in "Characterization of Municipal Solid Waste in the United States: 1990 Update", United States Environmental Protection Agency (EPA), Publication EPA/530-SW-90-042 dated June 1990.
A substantial portion of the total available MSW is being reduced by fire incineration either by mass burn or through combustion of refuse-derived fuel (RDF). While incineration remains controversial, it continues to find increasing acceptance due to a number of factors which include the greatly decreased amount of residual material which must be landfilled, and to improved operating procedures which emit lower concentrations of pollutants into the atmosphere. The use of incineration is increasing and as of 1990, over 160 incinerators were combusting about 10-15% of the MSW generated in this country.
The by-product of MSW incineration is ash, called "MSW ash". The ash represents about one-fourth of the mass of material prior to incineration. Generally, two types of incinerator systems are in use. The first type is called a mass burn system. These are large facilities, usually having a capacity of over 200 tons per day, which burn the unprocessed mixed MSW in a single combustion chamber, usually under conditions of excess air. A second system is one where the MSW is first processed by mechanical means to produce a more homogeneous fuel, It is known as refuse-derived fuel (RDF), which material is then combusted in a boiler, to form a residue in the form of ash.
Both major systems, as well as secondary or smaller systems known as modular systems, are capable of recovering energy from the burn, usually in the form of steam or electric energy, and produce ash as a solid waste by-product of combustion. The subject matter of this invention resides in the fixation and pelletization of products derived from MSW ash, which products meet 1990 Federal drinking water standards for toxins and hazardous materials, including heavy metals.
The term "ash" as used herein encompasses the gross residue from the incineration of MSW following an initial beneficiation to remove the gross or oversized, non-combustion or non-crushable objects. The gross ash content is classified as either "fly ash" or "bottom ash." Typically, the fly ash fraction amounts to from about 5-15% of the total ash and comprises lighter particles which are carried off the burning grate by the convection or turbulence, and condense or form in the flu gas system. Fly ash is removed by precipitators or collection bags in a baghouse. Fly ash can also include the superheater or economizer ash which collects on internal parts of the boiler system which are blown down or removed from time to time and combined with the fly ash fraction. The fly ash also frequently contains spent lime from an air quality control system (AQCS) in which a lime reagent is sprayed into the flue gases to neutralize sulfur dioxide and hydrochloric and hydrofluoric acids. The hot flue gases evaporate the water portion leaving a dry powder residue which is removed in a baghouse and combined with the fly ash. The AQCS fly ash may be combined with bottom ash, or maintained separately and stored in dry silos, depending on the particular plant operations.
"Bottom ash" is the coarse ash residue which accumulates on the grate. It usually falls directly into a water quench pit or tank from which it is removed to a storage area. When MSW is incinerated, a portion of the original material will be non-combustible and will emerge from the incineration process with the bottom ash. This residue contains such items as bottles, cans, rocks, metal, slag, and certain organic wastes, and for the purpose of this invention it is assumed that such gross material, as previously mentioned, is removed at a first classification, usually at the burn facility, for by-pass disposal, such as in landfill.
A consideration of the toxic and non-toxic residues in the ash requires an appreciation of the care which is taken, at the burn site, to segregate or to refuse delivery of unacceptable waste. For example, at the Hennepin County Waste-to-Energy Facility, Hennepin County, Minn., such items as explosives, pathological and biological wastes, radioactive material, incinerator residue, sewage and cesspool sludge, human and animal wastes, large motor vehicle parts, tires, farm machinery, transformers, trees, liquid wastes and other such wastes are refused entry, and thus do not form part of the MSW or its ash. Nevertheless, a broad spectrum of inorganic and organic compounds are necessarily subjected to incineration, and certain portions of these elements and compounds are found in the incinerator ash. While it may be possible for the ash to contain hazardous organic materials such as dioxins and furans, tests have shown that these are well below the levels considered harmful where the ash is collected from a facility which is properly operated to maintain a desired combustion temperature and combustion retention time.
Major inorganic components include aluminum, calcium, chlorine, iron, silicon, sodium and zinc as major components, along with carbon. The ash may also contain a broad range of trace metals, including the eight RCRA priority metals (As, Ba, Cd, Cr, Hg, PB, Se, Ag), as well s copper, cobalt, nickel and tin. In a typical facility, the major components may be identified in combined ash as shown in Table 1.
TABLE 1 ______________________________________ Silicon Dioxide 40% or greater Aluminum Oxide 10-20% Iron Oxide 10-20% Magnesium Oxide 1-6% Sodium Oxide 1-6% Potassium Oxide 1-6% Sulfate Ion 1-6% Chloride Ion 1-6% Phosphate Ion 1-6% ______________________________________
The ranges of the small or trace amounts of inorganic elements are represented by Table 2, in terms of pounds of material per ton of combined ash, representing a typical analysis.
TABLE 2 ______________________________________ CONCENTRATIONS OF INORGANIC ELEMENTS IN COMBINED ASH FROM MUNICIPAL WASTE INCINERATORS Elements Pounds/Ton of Ash ______________________________________ Arsenic * to 0.10 Barium 0.16 to 5.40 Cadmium * to 0.20 Chromium 0.02 to 3.00 Lead 0.06 to 73.20 Mercury * to 0.04 Selenium * to 0.10 Silver * to 0.19 Aluminum 10.00 to 120.00 Antimony &lt;0.24 to &lt;0.52 Beryllium * to * Boron 0.05 to 0.35 Calcium 8.2 to 170.00 Cobalt * to 0.18 Copper 0.08 to 11.8 Iron 1.38 to 267.00 Lithium 0.01 to 0.074 Magnesium 1.40 to 32.00 Manganese 0.03 to 6.26 Molybdenum * to 0.58 Nickel 0.03 to 25.82 Phosphorous 0.58 to 10.00 Potassium 0.58 to 24.00 Silicon 2.76 to 392.14 Sodium 2.20 to 66.60 Strontium 0.02 to 1.28 Tin 0.03 to 0.76 Titanium 2.00 to 56.00 Vanadium 0.03 to 0.30 Yttrium * to 0.02 Zinc 0.18 to 92.00 ______________________________________ (Excluding oxygen, sulfates and chlorine) *Less than 1/100 of a pound
Many of the inorganic constituents, or contaminants, have unlikely sources. Cadmium comes from metal coatings and platings on "white goods" such as home appliances, rechargeable batteries, printing inks and color pigments. Lead may originate in rust-proofing paints, wire and cable insulation, bottle caps, and the contact bases of burned-out light bulbs. Mercury is found in disposable batteries, such as hearing aid batteries, power control switches, certain paints, and fluorescent lights. Plastic materials are also a major source of lead and cadmium. Nickel-cadmium batteries include both nickel and cadmium.
Conventionally, the fly ash is combined with the bottom ash at the burn facility for transport to landfill. Only a small portion of the ash has found acceptance for commercial utilization, and in such instances where the resultant product is exposed to humans, there has been lacking an assurance that the material has been processed to recognized or particular EPA standards.
If the fly ash and spent lime fraction are combined with the bottom ash for treatment or disposal, the combined ashes will have moisture content of about 15-20%. The fly ash plus spent lime fraction is itself dry but is hygroscopic in nature and will absorb a portion of the moisture present in the bottom ash, thus reducing the overall combined ash content, when combined. The bottom ash moisture content is substantially higher, averaging 20-30%, where the fly ash is handled and stored separately, such as in dry bins.
Prior attempts at utilization, which have involved processing or treatment for hazardous materials or components, have generally failed to take advantage of the fact that the ash fractions themselves have differing concentrations of certain metals and other possible contaminants, and since they are formed and collected at physically separated locations, the fly ash fraction and the bottom ash fraction may be treated separately by processes tailored to the particular component or group of components in the fraction to be brought within required specifications.
In particular, the Resource Conservation and Recovery Act (RCRA) and the regulatory agencies operating under the Act (Public Law 94-580 (1976)) as Amended and as defined in Title 40 of the Code of Federal Regulations (40 CPR 142), have established maximum safe limits for solid waste and for drinking water as set out below in milligrams per liter:
TABLE 3 ______________________________________ Federal Drinking Water Limits as Mg/l Solid 1990 Federal Waste Drinking Water RCRA Metals Limits Standards ______________________________________ Arsenic 5.0 0.05 Barium 100.0 1.00 Cadmium 1.0 0.01 Chromium 5.0 0.10 Lead 5.0 0.05 Mercury 0.2 0.002 Selenium 1.0 0.01 Silver 5.0 0.05 ______________________________________
The drinking water standards for the eight RCRA priority metals set out above are 1/100th of that of the corresponding solid waste limits. Prior processes and techniques have not produced an economically useful product from MSW ash which meets federal drinking water standards when measured by the Toxicity Characteristic Leaching Procedure (TCLP) and as defined at 51 Fed Register 21648, Jun. 13, 1986 and in EPA Method 1311.
The TCLP leaching test has been found to be more aggressive than the extraction procedure toxicity test (EP-TOX) in the measurement of leaching potential, due to the lower pH (2.88) of the TCLP fluid #2. The repeatability of the results by different laboratories following the TCLP procedure makes it superior to the EP-TOX.
The TCLP procedure has now been adopted by the EPA as a replacement for the EP-TOX (EPA method 1310). The aggressiveness of the TCLP test requires that new approaches and methods for economic utilization of MSW ash be developed.
In order to make an economic use of a substantial part of the municipal solid waste ash produced at any given facility, it is necessary to process and convert the ash into a useable end product. If such product is used to add bulk to asphaltic or portland cement concrete mixes it may do so as an aggregate, but first must be converted to a suitable aggregate form, such as by pelletizing. Therefore, in addition to the fixation or chemical treatment of targeted components or elements, the end product must also meet those physical standards which have been created and promulgated for aggregates. These standards relate not only to sieve analysis and specific gravity, but also relate to other required properties of the product including hardness and durability in relation to resistance to abrasion, soundness and resistance to sulfate, resistance to freezing and thawing, and absorptivity. Thus, even though a synthetic by-product of MSW ash may be rendered inert by processing to the levels of federal drinking water standards, nevertheless such product would have restricted or limited commercial use if it failed to conform to the physical standards for such products.